The invasion and metastasis of cancer is a complex process which involves changes in cell adhesion properties which allow a transformed cell to invade and migrate through the extracellular matrix (ECM) and acquire anchorage-independent growth properties. Liotta, L. A., et al., Cell 64:327-336 (1991). Some of these changes occur at focal adhesions, which are cell/ECM contact points containing membrane-associated, cytoskeletal, and intracellular signaling molecules. The molecules contained within a focal adhesion include cytoskeletal proteins such as actin, paxillin, and tensin; ECM proteins such as fibronectin, laminin, and vitronectin; cell surface receptors such as the integrins; and protein tyrosine kinases such as src family kinases and a recently described tyrosine kinase, the focal adhesion kinase, or FAK.
The FAK gene was originally isolated from chicken and mouse fibroblasts and codes for a unique 125 kD cytoplasmic protein tyrosine kinase (p125.sup.FAK). Schaller, M. D., et al., Proc. Natl. Acad. Sci. USA 89:5192-5196 (1992); Hanks, S. K., et al., Proc. Natl. Acad. Sci. USA 89:8487-8491 (1992). The protein contains highly conserved consensus sequences within its tyrosine kinase domain, but is flanked by long amino-and carboxy-terminal sequences. It also lacks the src homology (SH2 and SH3) domains seen in the amino-terminal sequences of other cytoplasmic kinases. As FAK has begun to be characterized, a growing body of evidence suggests that FAK is a critical molecule in cell signaling events which regulate cell adhesion and motility, and may be of importance in the invasion and metastasis of cancer. Zachary, I. & Rozengurt, E., Cell 71:891-894 (1992). First of all, the activity of FAK is directly linked to the src oncogene. It has been demonstrated that p125.sup.FAK becomes phosphorylated, or activated, in cells which have been transformed with v-src, suggesting that FAK may play a role in the transformation by this oncogene. Recent data have shown that p60.sup.src stably associates with p125.sup.FAK, and it is postulated that the SH2 domain of src protects FAK from dephosphorylation by phosphatases, resulting in its constitutive activation. Cobb, B. S., et al., Mol. Cell. Biol. 14:147-155 (1994). The linkage of FAK to src is particularly intriguing, since levels of c-src activity have been shown to be increased in invasive and metastatic tumors. Weber, T. K., et al., J. Clin. Invest. 90:815-821 (1992); Talamonti, M. S., et al., J. Clin. Invest. 91:53-60 (1993). This raises the possibility that FAK may be a major downstream mediator of the invasive and metastic process in human tumors.
Another unusual property of FAK which suggests a role in invasion and metastasis is its relationship to the integrins and integrin-mediated signaling pathways. The integrin family of cell surface receptors have been shown to mediate many of the adhesive interactions of tumors and are now thought to be actively involved in signal transduction processes. Juliano, R. L. & Varner, J. A., Curr. Opin. Cell Biol. 5:812-818 (1993). The integrin molecules are composed of noncovalently bound .alpha. and .beta. subunits which link the cytoskeleton to the extracellular matrix by binding specific adhesion molecules such as fibronectin, talin, vinculin, and actin filaments. When cellular adhesion is mimicked by clustering integrin receptors with monoclonal antibodies or induced by plating cells on a fibronectin-coated substrata, increased phosphorylation of p125.sup.FAK has been demonstrated. Komberg, L., et al., J. Biol. Chem. 267:23439-442 (1992).
Specific integrin expression patterns have been associated with both cellular proliferation and metastasis. For example, overexpression of the .alpha.5.beta.1 integrin in human colon cancer cells has markedly reduced tumorigenicity in nude mice. Varner, J. A., et al., Mol. Biol. Cell 3:232A (1992). In contrast, other integrin expression patterns have been associated with invasion and metastasis, rather than cellular growth. Transfection of the .alpha.2.beta.1 integrin into the RD rhabdomyosarcoma cells has markedly increased tumor metastases in nude mouse tail vein injection assays. Chan, B. M. C., et al., Science 251:1600-1602 (1991). Furthermore, expression of either the .alpha.6.beta.4 laminin receptor or the .alpha.v.beta.3 integrin has been associated with metastatic behavior in studies of melanoma metastases. Ruiz, P., et al., Cell Adhesion Commun. 1:67-81 (1993), Gehlsen, K. R., et al., Clin. Exp. Metastasis 10:111-120 (1992). These findings further raise the possibility of a significant role for FAK in the metastatic process.
The final property of FAK which also suggests a link to cellular growth is its relationship to the growth stimulation of neuropeptides such as bombesin, vasopressin, and endothelin. These molecules exert mitogenic stimuli via receptors which are coupled to effectors via heterotrimeric G proteins. Stimulation of Swiss 3T3 cells with these neuropeptides has led to a rapid increase in specific p125.sup.FAK phosphorylation, suggesting that the effector molecules exert their stimuli via FAK. Zachary, I., et al., J. Biol. Chem. 267:19031-34 (1993). Thus, FAK appears to be a convergent pathway for growth stimulatory neuropeptides, transformation by the v-src oncogene, and integrin-mediated signaling.
Zachary, I. & Rozengurt, E., Cell 71:891-894 (1992).